Unit Three: Chemical Bonding and Intermolecular Forces

Lesson 2: Formation of Ionic Compounds

• Comptencies
• Starter Activities
• Main Activites

Competencies

• Define ionic bonding
• Describe the formation of an ionic bond
• Give examples of simple ionic compounds

Materials

• Charts of models of different ions and ionic compounds
• Two types of Styrofoam balls one of tennis ball size and the other about half of tennis ball size as many as possible

Note for the teacher;

This lesson is a lesson that should be delivered next to a lesson on ion formation in the sub-topic “formation of ionic compounds”; i.e the second lesson of the three lessons on ionic bond formation.

Start this lesson, by explaining that metal atoms give electrons and form cations, whereas non-metals lose electrons and form anions. These two attract each other by the electrostatic attraction force between oppositely charged ions to form ionic bond. Therefore, ionic compounds are made up of non directional bond in which all the oppositely charged ions attract each other in all directions and like charged ions repel each other. However, the common misconception here is that students believe that ionic compounds like NaCl are diatomic molecule. They fail to understand that ionic compounds are aggregates of ions. So, this lesson is designed to further clarify ionic bonding by using Styrofoam ionic models. The use of the model is not to teach the three dimensional crystal structure which is beyond the scope of the lesson but to describe that an ionic bond is non directional and the ionic compounds do not have molecular particles.

Starter activity (5minutes)

The lesson may be started by revising the formation of ions. You may write the symbols of some metals like Na, K, Mg and Al and that of the non-metals like Cl, F, O, and N; ask some students to write ionization reaction (i.e. lose of electron for the metal atoms to form cation and  gain of electron for the non-metals to form anion) , some of them to write electron configuration of the atoms and their ions, the rest to write Bohr’s models for the atoms and ions and finally Lewis/dot formula of the atoms and the ions. In this way you can involve many students in the revision activities.

It is also important to remind students about the interactions between opposite and like charges from their physics lessons.

Make sure that students successfully accomplish the above tasks as follows:

Evaluation      

Students may be asked the following questions

• How do metals form ions? What type of ions do they form? What are the ions called?
• How do non-metals form ions? What type ions do they form? What are the ions called?
• How big or small is the relative size of the cation as compared to its atom?      
• How big or small is the relative size of the anion as compared to its atom?
• What do we call the electron configurations of the cations and anions?
• With which elements’ configuration do they have similarity?
• What are the interactions between opposite charges? What about between like charges?

Main activity

Students may be asked to predict what happens when sodium and chlorine atoms are brought together. Let them discuss around their table, write down their impression and report to the class.

Some students may come up with correct answers from their previous knowledge of octet number and electron gain and lose. The others may be unable to bridge the last and the present concepts.

Using the more able students’ ideas you can assist the class to associate the concepts of formation of cations and anions and electrostatic attraction between oppositely charged particles.

Let students exercise writing formation of cations and anions and their attraction to form ionic bond using Bohr models, Lewis symbols and chemical reactions as shown in the starter activity table of this lesson.

Now ask students which structure they would choose for sodium chloride formed when atoms of sodium and chlorine combine.

Consider the smaller red sphere as sodium cation and the green larger one as chloride ion.
Which of the following is the most likely structure for sodium chloride?

D

Students may tend to choose “A” as a model for the structure of sodium chloride because probably they think that it is made up of separate molecules. However, it should be clear that each positive sphere or cation, attracts the opposite charged anions in all direction and the anions also attract the cations in all directions. As a result the number of anion surrounding the cation depends on the relative size of the ions. As there are forces of attractions between opposite charges and forces of repulsion between like charges, oppositely charged ions are placed as close as possible and the like charged ions as far apart as possible. In the case of sodium chloride each sodium ion is surrounded by six chloride ions and each chloride ion in tern, is surrounded by six sodium ions Such repetitive arrangement of the ions form a giant structure.

Now instruct the students to construct a three dimensional structure for sodium chloride using the atomic models or Styrofoam balls and a glue or gum.

Procedure

1. Colour the larger and smaller balls with different colours.
2. Take the small ball as a representing sodium ion and keep it on the table.
3. Paste the larger balls of chloride ion on the two sides, the top side at the bottom, the front and back sides of the sodium ball.
4. Do the same thing for each of the anions also by surrounding them by the cations.
5. Continue the construction with many balls as you have.

Evaluation

1. What is the ratio of the number of sodium ion to the number of chloride ions in sodium chloride?
2. How many sodium ions are there around each chloride ion?
3. How many chlorides are there around the sodium ions?
4. The following chart shows part of a periodic table.

Answer the following questions based on the table.

1. Which elements lose electrons to attain an octet of electrons in their outermost shells?
2. Which ones gain electron to attain octet number?
3. Element _________ loses three electrons to form a cation.
4. Element _________ gains three electrons to form an anion.
5. Write down the formulas of ionic compounds made up of the following pairs of elements.
   1. A and E
   2. H and D
   3. B and E
   4. C and G
   5. F and D
   6. C and E
6. Write down Lewis formula for the ionic compounds that would result from the combinations given in question (e) above

Answers to the questions

1. 1:1     2. 6     3. 6     4.  a.  A, B, C, H, F    b. G, D,E      c. C   d. G  

• i. AE    ii. H2D   iii. BE2     iv. CG     v. FD    vi. CE3

Concluding activity

Students may be asked to complete the following paragraph using the appropriate terms. Ionic bond is made by the electrostatic attraction force between (1) ________ and (2) ________ ions that are formed from (3) ________ elements and (4) _________ elements by losing and gaining electrons respectively. Ionic bond is also called (5) ________ bond. In ionic compounds all the anions are surrounded by (6) ________ and cations are surrounded by (7) ________; but the cations and anions depends up on the number of (8) ________ gained and lost. Ionic compounds can be represented by (9) ________ formula in which the dotes are used instead of  (10) _______ of the atoms.

Answer

1. catios
2. Anions
3. Metals
4. Non-metals
5. Electrovalent
6. Cations
7. Anions
8. Electrons
9. Lewis
10. Valence electrons